Hi Shannon! Congratulations to you guys on the marathon! It's going to be special!
The Epcot area resorts (Beach and Yacht Club, Boardwalk, Swan and Dolphin) would offer some viewing areas that would be within walking distance. If they were planning to enter Epcot, though, they would need theme park tickets. The only way to get to the viewing areas at the finish line (in the Epcot parking lots) is to go through the park. If they didn't want to be a the finish line, they could also view you as you run around the Boardwalk area in between Hollywood Studios and Epcot.
The monorail resorts (Polynesian, Grand Floridian, and Contemporary) would also offer some walking distance viewing areas, as the course will take you around the Seven Seas Lagoon that is in the middle of those resorts. My favorite viewing area for my family is for them to line up along Main Street U.S.A inside of Magic Kingdom. No theme park tickets are required for that, and the monorail could take them right to the entrance. Your kids may think it's pretty neat to be inside the Magic Kingdom before it even opens! And the gift shops on Main Street will be open to pass the time as they wait for you.
You can find a course map and spectator resource tools on the
Disney World Marathon website. Click on "event details" then, "course" for the maps. Or you can click on "spectator resources" for more information. Be sure to have them sign up for runner tracking so they will know where you are!
Let me know if you have any other questions!